Fusible resistor and mehtod of fabricating the same

ABSTRACT

A fusible resistor and method of fabricating the same is provided. The fusible resistor has a very low resistance of 20 to 470 mΩ by depositing thin films as a fusible element made of a material with low resistivity such as copper having a temperature coefficient of over 2,000 ppm/° C. The fusible resistor comprises a resistor body, a fusible element layer formed to surround the resistor body, caps formed to surround ends of the fusible element layer, lead wires attached to the caps, and an insulating layer for insulating the fusible element layer and the caps from outside. The thus-fabricated fusible resistor performs all functions of a use without generating excessive heat.

TECHNICAL FIELD

The present invention relates to a fusible resistor and method offabricating the same, and more particularly to a fusible resistor thatis inexpensive and has excellent electrical characteristics and methodof fabricating the same.

BACKGROUND ART

In general, fusible resistors are used to protect circuit elements ofelectronic devices. A fusible resistor functions as an ordinary resistorat normal loads, but as circuit breakers in an abnormal, overload state,due to its fusible characteristics.

Conventional fusible resistors are fabricated by coating a resistor bodywith a thin film made of a compound consisting of carbon, tin-nickel,and nickel-chrome by electroless plating and by performing a spiral cuton the surface of the coated resistor body (hereinafter, the spiralcutting will be referred to as “trimming”). While inexpensivefabrication of conventional fusible resistors is possible, manufacturinga fusible resistor with a resistance lower than 0.1 Ω is difficult dueto limitations of the manufacturing process. Further, fabricating afusible resistor with a resistance below 0.22 Ω is very difficult sincethe trimming causes an increase of the resistance of the fusibleresistor.

Where a current exceeding a predetermined range flows through thecircuit of an electronic device, a conventional fusible resistorgenerates excessive heat. To overcome this drawback, increasing therated current of a fusible resistor or using a micro fuse instead of thefusible resistor have been proposed. However, increasing the ratedcurrent results in an increase of the size of the fusible resistor.Further, using a micro fuse is not cost effective becausemass-production of micro fuses is limited due to the structuralcharacteristic of a micro fuse and expensive raw materials required.

DISCLOSURE OF THE INVENTION

Therefore, an objective of the present invention is to provide a fusibleresistor and method of fabricating the same, wherein the fusibleresistor is inexpensive and has excellent resistance and fusiblecharacteristics, without increasing the size of the fusible resistorwhen the rated current thereof is increased.

In accordance with one aspect of the present invention, there isprovided a fusible resistor comprising a resistor body; a fusibleelement layer, which surrounds the resistor body and is fusible when acurrent over a predetermined current value is applied to the resistorbody; caps, which surround ends of the fusible element layer; leadwires, which are attached to the caps; and an insulating layer forinsulating the fusible element layer and the caps.

In accordance with another aspect of the present invention, there isprovided a method of fabricating a fusible resistor comprising the stepsof: preparing a resistor body; forming a fusible element layer, whichsurrounds the resistor body and is fusible when a current over apredetermined current value is applied to the resistor body; formingcaps, which surround ends of the fusible element layer; forming leadwires, which are attached to the caps; and forming an insulating layerfor insulating the fusible element layer and the caps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1F are perspective views of each step of fabricating afusible resistor in accordance with a preferred embodiment of thepresent invention.

FIG. 2 is a graph of illustrating a measured temperature of aconventional fusible resistor and a fusible resistor in accordance withan embodiment of the present invention.

FIG. 3 is a graph of illustrating current-time characteristics of aconventional fusible resistor and a fusible resistor in accordance withan embodiment of the present invention.

MODES FOR CARRYING OUT THE INVENTION

A detailed description of the preferred embodiment of the presentinvention will follow with reference to the accompanying drawings.

As shown in FIG. 1A, conductive layer 2 made of a conductive material isdeposited on resistor body 1, which is in the form of a rod. In thisembodiment, resistor body 1 is made of a material such as a highly pureceramic. The conductive material of conduction layer 2 includesnickel-chromium and is deposited on resistor body 1 via plating, e.g.,an electroless plating, which has been used in conventional fusibleresistor fabrication.

Fusible element layer 3 having fusible characteristics is deposited onconductive layer 2 (FIG. 1B). Fusible element layer 3 fuses from theheat generated when an excessive current flows through resistor body 1.The temperature coefficient is a critical factor determining fusiblecharacteristics. Where a temperature coefficient is high, resistance offusible element layer 3 increases due to heat generated when a currentflowing through resistor body 1 is increased. As a result, thetemperature of fusible element layer 3 increases to the melting point sothat fusible element layer 3 is fused.

In this embodiment, a material including copper is used as fusibleelement layer 3. Copper is an electrically excellent fuse due to itshigh temperature coefficient, low resistivity, and low melting point.However, fusible element layer 3 may be made of any material, which hasa temperature coefficient of over 2,000 ppm/° C. and a resistivity of1×10⁻⁸ to 50×10⁻⁸ Ω·m (ohm meter).

Fusible element layer 3 may be deposited on conductive layer 2 viaelectrolysis plating. Instead of electrolysis plating, fusible elementlayer 3 may be directly deposited on resistor body 1 by sputtering.Where fusible element layer 3 is not deposited by electrolysis plating,conductive layer 2 may be omitted.

Anti-oxidation layer 4 is subsequently deposited on fusible elementlayer 3 in order to prevent oxidation of fusible element layer 3 in theatmosphere (FIG. 1C). For example, anti-oxidation layer 4 is formed byspray depositing a silver paste on fusible element layer 3. Instead ofdepositing anti-oxidation layer 4 on fusible element layer 3, aprotection layer made of, for example, silicon paint may be depositeddirectly on fusible element layer 3. However, using anti-oxidation layer4 is more advantageous since fusible element layer 3 may be oxidized inthe atmosphere during the fabrication process. Resistance of firststructure 10, which is comprised of three layers 2, 3, and 4, is definedby the kind and thickness of the materials consisting each of threelayers 2, 3, and 4. In this embodiment of the present invention, theresistance is extremely low, approximately, less than 5 mΩ.

As illustrated in FIG. 1D, a second structure 20 is constructed byforming caps 5 of a conductive material, such as iron, to wrap both endsof first structure 10. Fusible element layer 3 is electricallyconnected, through caps 5, to outside via anti-oxidation layer 4.Resistance of second structure 20 is maintained in range of 1 to 15 mΩ.

As illustrated in FIG. 1E, third structure 30 is constructed by forminga spiral groove 6, which penetrates layers 2, 3, and 4. Final resistanceof third structure 30 is conventionally maintained in the range of 20 to470 mΩ. This final resistance depends on the resistance of firststructure 10 and the number of trimming turns. More particularly, thefinal resistance after trimming depends on the number of trimming turns.The number of trimming turns is determined as 1 to 2. According to theresistance dependent on the number of trimming turns, characteristicswith respect to rated currents of fuse and the like are determined.

Finally, as illustrated in FIG. 1F, lead wire 7 is attached to an end ofeach of caps 5 by welding. Lead wire 7 electrically connects a circuitsubstrate to fusible element layer 3, wherein a resultant fusibleresistor is to be installed on the circuit substrate. The resultantfusible resistor, i.e., fusible resistor 40 is constructed by coating anoutside of third structure 30 with an insulating paint to formprotective film layer 8. Herein, protective film layer 8 isolatesfusible element layer 3 and caps 5 from the outside and protectscomponents 1 to 6 within fusible resistor 40 from external impacts. Anouter surface of protective film layer 8 is preferably formed of anoncombustible paint so as to indicate a rated current and the like offusible resistor 40.

Referring to FIG. 2,in measuring temperature, a conventional fusibleresistor fabricated by Smart Electronics, Inc. in Korea (Model No. FNS2W, rated current of 2 watt (W), resistance of 0.47 Ω, 12 mm in lengthexcept lead wires) and a fusible resistor in accordance with anembodiment of the present invention fabricated by Smart Electronics,Inc. in Korea (Model No. SPF 1W, rated current of 1W, resistance of 0.02Ω, and 6.5 mm in length except lead wires) are compared. Temperature ismeasured by coupling a temperature sensor to the lead wires and sensing,every 5 minutes, the temperature of each fusible resistor where acurrent of 2.5A is applied thereto. In measuring temperature, atemperature sensor of Yokogawa Electric Corporation in Japan (Model No.μ1800) is employed.

As illustrated in FIG. 2, heat generated on the conventional fusibleresistor rises from 27.5° C. to 105.8° C. after 5 minutes to reach112.2° C. after 1 hour, while temperature of the fusible resistor inaccordance with an embodiment of the present invention rises from 27.5°C. only to 34.8° C. after 5 minutes to reach only 36.1° C. after 1 hour.

In general, the temperature of the fusible resistor falls as its ratedcurrent increases. However, in accordance an embodiment of the presentinvention, the temperature and its range of the fusible resistor areremarkably lower than those of a conventional fusible resistor, in spiteof having a rated current lower than that of the conventional fusibleresistor. With the above advantageous features, the fusible resistor inaccordance with an embodiment of the present invention is directlymounted on a circuit substrate to reduce the size of an electronicdevice.

Referring to FIG. 3, the dotted line and solid line representcurrent-time characteristics with respect to the resistance of theconventional fusible resistor and the fusible resistor in accordancewith an embodiment of the present invention, respectively. The fusibleresistors used for measurement of current-time characteristics areidentical to those used for measurement of temperature, described abovewith reference to FIG. 2.

INDUSTRIAL APPLICABILITY

As described above, the present invention provides a fusible resistorhaving a very low resistance, e.g., from 20 to 470 mΩ, by depositing afusible element layer made of a material such as copper, which has atemperature coefficient of 2,000 ppm/° C. and low resistivity, on aresistor body. A fusible resistor in accordance with an embodiment ofthe present invention having low resistance does not overheat during anoverload.

Thus, a fusible resistor in accordance with an embodiment of the presentinvention can be used for blocking an excessive current induced byinstantaneous short phenomenon of a diode, a capacitor, and a transistorin an excessive current preventing circuit. Further, such a fusibleresistor can be replaced by a conventional resistor having a resistanceof 0.1 to 2 Ω, depending on the minimum current of each wire on anelectronic circuit. Furthermore, the method of fabricating the fusibleresistor in accordance with the present invention can be implementedwithout additional investment of equipment for manufacturing the fusibleresistor since it adapts conventional fabricating methods. Accordingly,the fabricating method in accordance with an embodiment of the presentinvention has high productivity.

While the present invention has been shown and described with respect tothe particular embodiments, those skilled in the art will recognize thatmany changes and modifications may be made without departing from thescope of the invention as defined in the appended claims.

1. A fusible resistor, comprising: a resistor body; a fusible elementlayer, which surrounds the resistor body and is fusible when a currentover a predetermined current value is applied to the resistor body;caps, which surround ends of the fusible element layer; lead wires,which are attached to the caps; and an insulating layer for insulatingthe fusible element layer and the caps.
 2. The fusible resistor of claim1, wherein the fusible element layer further comprises at least copper.3. The fusible resistor of claim 1, wherein the fusible element layerfurther comprises a material having a temperature coefficient of over2,000 ppm/° C. and a resistivity of 1×10⁻⁸ to 50×10⁻⁸Ω·m (ohm/meter). 4.The fusible resistor of claim 1, further comprising an anti-oxidationlayer, which surrounds the fusible element layer.
 5. The fusibleresistor of claim 4, wherein the anti-oxidation layer further comprisesat least a silver paste.
 6. The fusible resistor of claim 1, furthercomprising a conductive layer, which is formed between the resistor bodyand the fusible element layer and made of a conductive material.
 7. Thefusible resistor of claim 4, wherein the conductive layer furthercomprises at least nickel and chrome.
 8. The fusible resistor of claim6, further comprising a groove, which is formed through the fusibleelement layer, the anti-oxidation layer, and the conductive layer toreach the resistor body.
 9. The fusible resistor of claim 8, wherein thegroove is in the form of a spiral along a circumference of the fusibleresistor.
 10. A method of fabricating a fusible resistor, comprising thesteps of: preparing a resistor body; forming a fusible element layer,which surrounds the resistor body and is fusible when a current over apredetermined current value is applied to the resistor body; formingcaps, which surround ends of the fusible element layer; forming leadwires, which are attached to the caps; and forming an insulating layerfor insulating the fusible element layer and the caps.
 11. The method ofclaim 10, wherein the fusible element layer further comprises at leastcopper.
 12. The method of claim 10, wherein the fusible element layerfurther comprises a material having a temperature coefficient of over2,000 ppm/° C. and a resistivity of 1×10⁻⁸ to 50×10⁻⁸Ω·m (ohm/meter).13. The method of claim 10, further comprising a step of forming ananti-oxidation layer, which surrounds the fusible element layer.
 14. Themethod of claim 13, wherein the anti-oxidation layer further comprisesat least a silver paste.
 15. The method of claim 10, further comprisinga step of forming a conductive layer, which is formed between theresistor body and the fusible element layer and made of a conductivematerial.
 16. The method of claim 15, wherein the conductive layerfurther comprises at least nickel and chrome.
 17. The method of claim15, further comprising a step of forming a groove, which is formedthrough the fusible element layer, the anti-oxidation layer, and theconductive layer to reach the resistor body.
 18. The method of claim 17,wherein the groove is in the form of a spiral along a circumference ofthe fusible resistor.
 19. The fusible resistor of claim 4, furthercomprising a conductive layer, which is formed between the resistor bodyand the fusible element layer and made of a conductive material.
 20. Themethod of claim 13, further comprising a step of forming a conductivelayer, which is formed between the resistor body and the fusible elementlayer and made of a conductive material